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Spotted knapweed, Centaurea maculosa Lam. (Compositae) is an extremely
successful introduced weed species in the northwestern United States. Multiple factors
may be responsible for its success, including chemical defense. Two different types of
putative chemical defense agents are the focus of this thesis. The first is a bitter-tasting
sesquiterpene lactone, cnicin, which occurs at high concentrations in foliar tissues; the
second is a group of polyacetylenic compounds produced in the roots. This study
represents the first designation of structural identities to polyacetylenic compounds from
North American populations of C. maculosa. Both cnicin and polyacetylenic compounds
have demonstrated biological activity, suggesting roles in chemical defense. In order to
test predictions of chemical defense hypotheses for this plant, concentrations of cnicin
and polyacetylenes were measured in a greenhouse experiment in response to varying soil
nitrogen and light availability. Light and soil nitrogen availability were important in
determining concentrations and content of cnicin, whereas polyacetylene concentrations
were not significantly affected by differences in these environmental parameters. Cnicin concentrations were also measured in a field experiment in response to varying levels of
soil nitrogen and water. No significant differences in cnicin concentrations were
observed among treatments in either young (rosette-stage) or mature aerial tissues.